Cyber Security Notes

Wireless Security

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tagscybersecurity, concept, csf-ns, wireless, wpa2, wpa3, wifi-attacks, unit-3

8 min read

Wireless Security

📝 Concept Note — CSF & NS Unit III Deep Dive | Source: MCA Sem III Syllabus

The Problem It Solves

Wired networks have a physical security property: to attack them you must plug something in. Wi-Fi broadcasts in every direction through walls and out into the car park. Anyone within radio range can passively capture every frame — no compromise required.

So wireless security cannot rely on physical access control. It must assume the attacker is already listening and solve everything with cryptography and authentication.

The Evolution — and why each generation broke

WEP (1997)WPA (2003)WPA2 (2004)WPA3 (2018)
CipherRC4RC4 + TKIPAES-CCMPAES-GCMP
Key exchangeStatic key4-way handshake4-way handshakeSAE (Dragonfly)
Forward secrecy❌❌❌✅
StatusBrokenDeprecatedVulnerable (KRACK) but patchableCurrent
  • WEP failed because its 24-bit initialisation vector (IV) was far too short. On a busy network IVs repeat within hours, and reused keystream lets an attacker recover the key. WEP is crackable in minutes — never acceptable.
  • WPA was a stopgap on the same hardware: TKIP rotated keys per packet, but kept RC4 and is now broken too.
  • WPA2 moved to AES-CCMP — the cipher itself remains sound. Its weakness is the handshake (KRACK) and offline cracking of weak passphrases.
  • WPA3 fixes the handshake itself.

⚙️ How It Actually Works — the WPA2 4-Way Handshake

This is the heart of Unit III’s wireless section. The goal: both sides prove they know the password and derive fresh session keys — without ever sending the password over the air.

Starting point: both the client and AP already know the PMK (Pairwise Master Key), derived from the passphrase + SSID via PBKDF2. The PMK itself is never transmitted.

CLIENT (supplicant)                         ACCESS POINT (authenticator)
        │                                              │
        │  ◄─── (1) ANonce ─────────────────────────── │  AP sends random number
        │                                              │
   Now client has: PMK + ANonce + SNonce + both MACs   │
   → derives the PTK (session key)                     │
        │                                              │
        │  ─── (2) SNonce + MIC ──────────────────────►│  client's random + proof
        │                                              │
        │                          AP now derives the same PTK,
        │                          verifies MIC → client knew the PMK ✅
        │                                              │
        │  ◄─── (3) GTK (encrypted) + MIC ──────────── │  group key for broadcast
        │                                              │
        │  ─── (4) ACK ───────────────────────────────►│
        │                                              │
        └────────── encrypted session begins ──────────┘

The pieces:

  • ANonce / SNonce — random “numbers used once” from AP and client. They guarantee every session derives different keys, even with the same password.
  • PTK (Pairwise Transient Key) — the actual session key encrypting your unicast traffic. Derived as PTK = f(PMK, ANonce, SNonce, AP MAC, client MAC). Because both sides have all five inputs, both compute the same PTK independently — it is never transmitted.
  • MIC (Message Integrity Code) — proves the sender knows the PTK (and therefore the PMK/password) without revealing it.
  • GTK (Group Temporal Key) — shared by all clients, used for broadcast/multicast; re-keyed when a device leaves.

Why this handshake is attackable

The handshake is sent in the clear (it must be — no keys are established yet). An attacker who captures all four messages gets the ANonce, SNonce, MACs, and MIC. They cannot reverse the MIC, but they can guess offline:

for each candidate password:
    PMK  = PBKDF2(password, SSID)
    PTK  = f(PMK, ANonce, SNonce, MACs)
    if computed MIC == captured MIC:  → password found

This is offline brute force — unlimited attempts, no interaction with the network, no lockout. The only defence is a long, high-entropy passphrase. A dictionary word falls in seconds; a 20-character random passphrase is infeasible.

💡 This is why attackers send deauth frames: to force a reconnection so they can capture a fresh handshake. (Also why PMKID attacks matter — on some APs a single frame yields crackable material without waiting for any client.)

WPA2-Personal vs WPA2-Enterprise

Personal (PSK)Enterprise (802.1X)
CredentialOne shared passphrase for everyonePer-user credentials/certificates
Auth backendNoneRADIUS server + EAP
Revoking one userChange password for everyoneDisable that one account
Attacker who knows the passwordCan decrypt others’ traffic (with handshake)Cannot — keys are per-user
UseHome, small officeAny real organisation

802.1X flow: the client (supplicant) authenticates through the AP (authenticator) to a RADIUS server (authentication server). The AP passes traffic only after the server approves, and a unique PMK per user per session is issued. EAP variants: EAP-TLS (certificates both sides — strongest), PEAP/EAP-TTLS (server cert + user password).

WPA3 — how it fixes things

SAE (Simultaneous Authentication of Equals), a.k.a. Dragonfly, replaces the PSK handshake:

  • It is a password-authenticated key exchange. The captured exchange contains no material that can be brute-forced offline — each password guess requires a fresh live interaction with the AP, which can be rate-limited and logged.
  • Forward secrecy: each session uses fresh ephemeral keys. Capturing traffic today and learning the password later still doesn’t decrypt it. (Under WPA2 it does — a critical difference.)
  • Protected Management Frames (802.11w) are mandatory, which kills classic deauth attacks.
  • OWE (Opportunistic Wireless Encryption) for open networks: encrypts café/airport Wi-Fi individually per client even with no password. Not authentication — but it stops trivial passive sniffing.

Common Wi-Fi Attacks

Evil Twin

Attacker runs a rogue AP broadcasting the same SSID as a legitimate one, often with a stronger signal. Devices prefer the stronger signal and auto-connect (especially to remembered networks). All victim traffic then flows through the attacker.

Usually paired with a captive portal that phishes credentials. Defences: always use HTTPS/VPN, disable auto-connect to open networks, use 802.1X (the client validates the server certificate, so a fake AP fails), and WIPS to detect rogue APs.

Deauthentication attack

In WPA2, management frames are unauthenticated. Anyone can forge a “deauthenticate” frame claiming to be the AP, and the client obeys and disconnects.

Used to (a) deny service, or (b) — more importantly — force a reconnection to capture the 4-way handshake for offline cracking, or push victims onto an evil twin.

Defence: 802.11w Protected Management Frames (PMF), which authenticates management frames. Optional in WPA2, mandatory in WPA3.

KRACK (Key Reinstallation Attack, 2017)

Targets the handshake protocol logic, not the AES cipher.

Mechanism: message 3 of the handshake may be retransmitted if the AP doesn’t receive an ACK. An attacker in a MitM position replays message 3. The client reinstalls the already-in-use PTK, which resets the nonce/packet-number counter to zero. Reusing a nonce with the same key means the same keystream encrypts different plaintexts — enabling decryption and, in some cases, packet forgery.

Key points: it did not break AES and does not reveal the Wi-Fi password. It was fixed in software — patch clients and APs. Android/Linux (wpa_supplicant) were worst hit because they reinstalled an all-zero key.

Others worth knowing

  • WPS PIN — the 8-digit PIN is validated in two halves, reducing brute force to ~11,000 attempts. Always disable WPS.
  • Rogue AP — an employee plugging in an unauthorised AP, bypassing all perimeter controls.
  • Karma/probe attacks — devices broadcast probes for remembered SSIDs; an attacker answers “yes, that’s me”.

Securing a Wireless Network

  1. Use WPA3 where supported; otherwise WPA2-AES (never WEP/TKIP).
  2. Long random passphrase (20+ chars) — the only defence against offline cracking on PSK.
  3. WPA2/WPA3-Enterprise (802.1X) for any organisation — per-user credentials and revocation.
  4. Disable WPS.
  5. Enable 802.11w (PMF) to stop deauth.
  6. Change default admin credentials and SSID; keep firmware patched (KRACK et al.).
  7. Separate guest and IoT SSIDs onto isolated VLANs (DMZ & Network Segmentation).
  8. Deploy WIDS/WIPS to detect rogue and evil-twin APs.
  9. Reduce transmit power / place APs so coverage doesn’t flood the car park.

⚠️ Not real security: hiding the SSID (it’s still in probe/beacon traffic) and MAC filtering (MACs are sent in cleartext and trivially spoofed). Know these as classic exam distractors.

⚠️ Gotchas / Exam Traps

  • The PMK/password is never transmitted in the 4-way handshake — the MIC proves knowledge of it.
  • Capturing a handshake enables offline brute force; passphrase strength is the whole defence.
  • KRACK broke the handshake logic (nonce reuse), not AES, and does not disclose the password.
  • Deauth works because WPA2 management frames are unauthenticated → fixed by 802.11w/WPA3.
  • WPA3’s SAE gives forward secrecy; WPA2 does not — learning the password later decrypts old WPA2 captures.
  • SSID hiding and MAC filtering are not security controls.

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